Gridded, broad-beam ion sources, first developed for ion propulsion engines for spacecraft, are used in a variety of applications, such as ion beam etching (IBE), ion beam sputter deposition (IBSD), materials modification, and nuclear fusion technology. Ions are usually extracted from a discharge plasma by multi-grid ion optics. The plasma generator and the ion optics assembly are the two major components of the broad-beam ion source.
The plasma is usually generated by a type of high voltage glow discharge, hot-cathode discharge, vacuum arc discharge, or RF discharge. Ions extracted from the plasma are accelerated and focused into an ion beam by applying relevant potentials to an electrode in contact with the plasma and other grid electrodes (ion optics). The optimum number of grid electrodes is defined by application requirements, such as cost, weight, sensitivity to contamination of exposed surfaces by grid material, and beam collimation.
For many ion beam etch and ion beam sputter deposition applications, grid assemblies which provide low ion beam divergence are needed. Grid assemblies using three or more grid electrodes are preferable for this purpose. Such grid assemblies are able to provide low beam divergence over a wide range of beam current and beam voltage (ion energy). In addition, when operated under proper conditions, grid assemblies with three or more grid electrodes are not subject to grid erosion from charge exchange ions generated in the ion beam. For comparison, one and two-grid systems are mechanically simpler but have a limited range of operation at low beam divergence and are subject to grid erosion. Consequently, three-grid ion optics, with longer grid life, are more compatible with high purity materials processing requirements.
In a three-grid assembly, the grid in contact with the plasma is conventionally called the screen grid, and has a positive potential close to the plasma potential that defines the ion energy. The next grid downstream in the beam usually is set at a negative potential, and is called an accelerator grid. For low beam divergence operation, the absolute value of accelerator potential should not be greater than 0.3 times the value of the screen grid potential. The third grid is most commonly connected to ground potential, as are the target and chamber components. The third grid is called the decelerator grid.
Ion Optics Design and Operational Considerations
In a majority of broad ion source applications for high throughput production processes (or high thrust ion engines), the plasma generator and the multi-grid optics assembly must provide high beam current density at the ion optics and beam target. In turn, the maximum ion beam current (Ib) is very sensitive to the total extraction voltage Vt, which is sum of absolute values of screen potential (Vs) and accelerator potential (Va), and the spacing between these grids (d). To a good degree of accuracy this dependence can be expressed by the Child-Langmuir equation, Ib˜(Vt)3/2/d2. As illustrated, ion beam current is inversely proportional to the square of the grid spacing; smaller grid spacing produces significantly higher ion beam current.
In the technologies mentioned above, (IBE, IBSD, and ion thruster applications), the ion energy is relatively low and usually does not exceed approximately a few kilovolts. To achieve high ion beam densities and low beam divergence, the inter-electrode spacing in the grid assembly must be on the order of 0.5-2.0 mm. This small spacing must be maintained over large beam diameters, up to 50 cm and more. Furthermore, technological requirements for ion beam uniformity (1% or less) and beamlet divergence (less than 3-5 degrees half-angle) dictate tight tolerances for grid inter-spacing and hole alignment. Grid inter-spacing tolerance is typically ±0.050 to 0.10 mm over the entire grid assembly. Grid hole misalignment is maintained at less than 0.05 mm with a 2 mm grid hole diameter. Maintaining these tight tolerances requires strict manufacturing control coupled with exceptionally stable grid structures and mounting configurations. Providing and maintaining these demanding tolerances is substantially complicated by thermal gradients which can exist between the center and periphery of the grids and also between the grid electrodes and grid support components.
In addition to the need for accurate grid inter-spacing during manufacture, the ion optics are also subject to repeated operational thermal cycling between “hot” (plasma on) and “cold” (plasma off) states. In a design where the mounting portion of the grid assembly is placed outside of the plasma generator, the temperature gradients are great. It has been observed in this configuration for the temperature of the center portion of the grids as much as 200 degrees Celsius higher than the temperature of the outer diameter of the grids.
Different techniques have been proposed to improve the thermal and mechanical stability of grids. These techniques include holding the grids in tension, supporting the screen grid in its center by a post contained in the plasma generator, adding stiffening ribs and using inter-electrode support spacers. However, at present, a common technique to provide stability is by forming the grid electrodes in a dished hemispherical configuration. As a result of the three dimensional shape, a dished grid has different mechanical stability when compared to a flat grid. A dished grid also has different thermal characteristics when compared to a flat grid. One difference is that thermal deformation of a dished grid is more predictable in magnitude and direction.
In addition to different thermal and mechanical characteristics when compared to flat grids, dished grid assemblies are more appropriate for special applications where highly focused or defocused ion beams are required. Concave grids (where the dishing is toward the plasma source) produce a focused ion beam that can be used in ion beam sputter deposition systems with relatively small target areas and high density ion beams. On the other hand, convex grids (where the dishing is away from the plasma source) produce defocused beams used in ion beam processing systems, such as in substrate surface cleaning, when a relatively large substrate or target area is exposed to a low density ion beam.
With a flat peripheral area on the grid (either flat or dished grid with an outer flange), it is known to use relatively massive stiffening ring arrangements to support and stiffen the grid. These stiffening rings are also usually fabricated from the same material as the grids, and are fastened to the flat peripheral area of the grid. In turn, the grid stiffening rings are fastened to each other and/or to the grid mounting base with some form of fastener. The fasteners are varied and include rigid posts, screws, nuts, washers, insulating bushings, and “sputter cups.” “Sputter cups” protect insulator surfaces from shorting out due to deposition of conductive materials. Because these designs have multiple parts and tend to be somewhat complex, they usually require some manual grid alignment, at least for initial set-up.
However, grid stiffening rings are exposed to rapid thermal transitions. It is commonly known that the relatively massive rings can introduce larger temperature gradients in the radial direction. In addition, if there is poor thermal contact between the edge of the grid electrode and the stiffening ring, a transient azimuthal temperature variation will occur. Nonuniformity in the temperature distribution can lead to grid distortion with consequent aperture misalignment and beamlet vectoring, which can cause ion impingement on the accelerator and decelerator grids. Finally, utilization of molybdenum stiffening rings appreciably increases the construction weight and cost.
It is also known to isolate the grid from the support structure with flexible supports. Presumably in this configuration, grid deformation is less influenced by the support structure. However, flexible grid margins and grid supports do not completely avoid problems associated with grid thermal expansion. During operation, radially and axially acting elastic forces can deform the grids and cause grid spacing deviation and misalignment.
It is also known to incorporate a “dog leg” or other bend profiles into the planar edge of the grid electrodes. Presumably, this is done to provide stiffness.
It is also known to profile the outer rims of the grids into a trapezoidal shape to define a space between them for insulating spacers. The flat portion of the profiled grid rims has plurality of slots. Ball-shaped insulating spacers are seated in these slots. Presumably, this configuration provides alignment of the grids and allows radial expansion.
It is also known to reduce the number of grid supports in an attempt to isolate the grid from the support. However, use of a limited number of grid supports (rigid or flexible) does not necessarily provide stable and uniform grid spacing as a function of the azimuthal angle.
Manufacture and Assembly Considerations for Ion Optics
The majority of dished grid optics are fabricated from cross rolled molybdenum sheets. The thickness of the sheet material depends on the hole machining technology used to form the grid. For conventional photochemical sheet etching to produce the grid hole pattern, the sheet thickness is in the range 0.2-0.5 mm. When the grid hole pattern is produced by drilling the sheets, the grid can be thicker. However, grid thickness is balanced with restrictions dictated by the ion optics and grid dishing technology. For some production applications, such as high rate ion beam sputter deposition, grid sheet material thickness in the range of 1 mm is practical.
In an operation that is separate from creating the grid hole pattern, the grid is typically dished by physical deformation. Numerous techniques have been employed to accomplish this deformation, including hammering, spinning, cold and hot pressing, and hydroforming.
For most grid deformation techniques to form a dished grid, a specially designed fixture is used to clamp the peripheral edges of the molybdenum grid sheets while deforming the center. One reason for clamping the edges is to keep the peripheral area of the dished grid as flat as possible. This region of the grids is typically used as part of the grid mounting.
When a grid is dished by clamping the periphery and deforming the center, the undished periphery is often distorted when the grid is unclamped. Depending on the dishing technology and condition (clamping method and force, grid and fixture temperatures and temperature gradients, material thickness, etc.) the periphery or flange is bent either toward or away from the grid central axis. In addition, the periphery can become wavy (uneven). Distortions extending into the periphery of the spherical surface of the dished portion of the grid have also been observed. A reason for distortion is the “spring back” which is caused by the internal stresses left in the transition region between the clamped peripheral flange and the dished spherical surface. To restore the spherical shape to the dished surface and flatness to the peripheral flange, the grids are typically stress-relieved. During the stress-relieving operation the grids are clamped in a fixture with spherical shaped dies and components made of a high temperature compatible material, such as graphite. This procedure does not always provide satisfactory results. If stresses are not fully relieved, they can be induced in the electrodes when they are clamped between the mounting rings or fastened to the flexible grid supports. Such stresses can lead to changes in the grid-to-grid spacing and hole misalignment under the thermal stress of ion source operation.
In most known grid assemblies, a plurality of components are used. These components define inter-electrode spacing and hole alignment. During production the plurality of components leads to tolerance buildup problems with associated inter-grid spacing deviations and hole misalignment.
In production applications of gridded ion sources, a critical requirement is grid hole alignment. Maintenance of the ion source is substantially simplified if the grid assembly leads to the proper hole alignment (e.g., a self-aligning grid assembly). Grid assembly designs using either massive and rigid supports, or flexible supports typically are not self-aligned constructions.
It is therefore an objective to provide a grid assembly design that is adaptable to both flat and dished grids. The design should avoid or reduce problems with manufacturing and operating stress in the peripheral region of a dished grid, and the design should be conducive to self-alignment of the individual elements of the assembly. The design should minimize the number of individual parts in the grid assembly. The design should allow use of lower cost materials. The design should include features to allow extended operational time without the need for cleaning or maintenance. The design should include materials that allow extended operational time without the need for cleaning or maintenance. The design should allow extended operation without the need for replacement of parts. The design should be scalable to support larger beams without compromising accuracy and performance.